Install a proper antivirus and stop installing all the crap applications, video codec packs, toolbars and so on.

Use a player like Media Player Classic Home Cinema to play videos without any need to install codecs.
Use Firefox or Chrome to browse the net safely.
Use a proper antivirus like Eset NOD32 or Avira or whatever else is cool these days.

Set your Windows' page file to a fixed size of 3-4 GB and after a reboot, defragment your windows drive using a good software like o&o defrag.